Coarse bubble diffusers produce large bubbles, and are designed for use in applications where mixing energy is prioritized over oxygen mass transfer, and where routine maintenance is problematic, as coarse bubble diffusers have traditionally been viewed by the market as being maintenance free, or at least less maintenance intensive than fine bubble diffusers. Applications for these products include primary channels, equalization basins, aerobic digesters, sludge storage tanks, post aeration basins, aeration basins with media requiring scouring or sloughing of biofilm, and some aeration tanks where energy costs may be low or capital is not available to change to fine bubble systems.
In recent years, there has been a trend to cycle diffusers on and off to save power or to provide mixing with less oxygen transfer, for example, to cultivate anoxic bacteria, which prefer to live in an oxygen-poor environment. Batch processes may also require intermittent aeration, as these often fill, aerate, and decant in the same tank in three distinct phases, not all of which use aeration. Greenfield plants have also been built in developing countries with interrupted power supplies, resulting in brownouts, which inadvertently shut off even those systems designed to operate continuously. New methods of sludge dewatering, such as advanced centrifuges, have resulted in higher solids concentrations in aerobic digesters and sludge storage tanks.
Popular coarse bubble diffusers on the market, like the wide band type, have no check valve, so when compressed gas is turned off, the diffuser and the piping system may flood with sludge. If that sludge is thick, or if that sludge remains in the piping system and the diffuser for an extended period of time, it has a tendency to dry out and clog the diffuser and piping system. Rags, sand, grit, and snails are also problems for diffusers that flood when the gas supply is stopped.
Other coarse bubble diffusers like the three-inch and five-inch diameter membrane caps, provide the appearance of having a check valve, but in fact they do not provide a strong check valve function in many applications. In these membrane caps, there is a flap of rubber without holes in it designed to rest over the gas orifice when compressed gas is turned off, but typically the orifice is large, and the slightest latent pressure in the piping system is usually enough to lift a membrane above the orifice where sludge can flow back into the piping system. As this membrane ages, shrinks, or stretches, its resistance to back-flow lessens even more. This diffuser is flat at rest, and is stretched by design in operation. After the system has been at rest for some time the piping system fills with process fluid and sludge. When the compressed gas is turned back on, the membrane acts like a filter, keeping most of the solids in the diffuser and in the pipe, drying them out, and allowing the liquid back out. Initially the piping system when flooded contains mostly liquid and some sludge, but over time the sludge packs itself into the piping, often requiring high pressure hoses to clean it out.
To try to deal with this, a “duckbill” type coarse bubble diffuser has been developed that is called a “check valve” coarse bubble diffuser, and is described in U.S. Pat. No. 6,017,839 to Raftis et al., entitled “Gas Diffuser Valve.” However, this type of diffuser can suffer back flow and maldistribution, as it relies on rubber against rubber sealing when the gas is shut off. Rubber is prone to stretching and shrinking over time, particularly in a design which stretches by design as it operates, and as this occurs, a pathway can open up for sludge to enter the diffusers and piping system. Accordingly, for proper performance, these diffusers need to be replaced on a schedule. Failing to do that can result in very high system pressures, as the gas has to overcome the resistance of clogged pipes and diffusers. Furthermore, this type of diffuser, like most diffusers, has a fixed control orifice. This means it is able to distribute gas uniformly only in a tight band of gasflow rates per diffuser. Provide too little gas to the grid and typically only the diffusers closest to the gas supply pipe open up and operate; provide too much gas and the orifice headloss rises exponentially with flow.